Some micro-mechanical systems (MEMS) devices process, or use liquids to operate. In one class of these liquid-containing devices, resistive heaters are used to heat the liquid to the liquid's superheat limit, resulting in the formation of a rapidly expanding vapor bubble. The impulse provided by the bubble expansion can be used as a mechanism for moving liquid through the device. This is the case in thermal inkjet printheads where each nozzle has a heater that generates a bubble to eject a drop of ink onto the print media. In light of the widespread use of inkjet printers, the present invention will be described with particular reference to its use in this application. However, it will be appreciated that the invention is not limited to inkjet printheads and is equally suited to other devices in which vapor bubbles formed by resistive heaters are used to move liquid through the device (e.g. some ‘Lab-on-a-chip’ devices).
The resistive heaters in inkjet printheads operate in an extremely harsh environment. They must heat and cool in rapid succession to form bubbles in the ejectable liquid—usually a water soluble ink with a superheat limit of approximately 300° C. Under these conditions of cyclic stress, in the presence of hot ink, water vapor, dissolved oxygen and possibly other corrosive species, the heaters will increase in resistance and ultimately go open circuit via a combination of oxidation and fatigue, accelerated by mechanisms that corrode the heater or its protective oxide layers (chemical corrosion and cavitation corrosion).
To protect against the effects of oxidation, corrosion and cavitation on the heater material, inkjet manufacturers use stacked protective layers, typically made from Si3N4, SiC and Ta. In certain prior art devices, the protective layers are relatively thick. U.S. Pat. No. 6,786,575 to Anderson et al (assigned to Lexmark) for example, has 0.7 μm of protective layers for a ˜0.1 μm thick heater.
To form a vapor bubble in the bubble forming liquid, the surface of the protective layers in contact with the bubble forming liquid must be heated to the superheat limit of the liquid (˜300° C. for water). This requires that the entire thickness of the protective layers be heated to (or in some cases above) the liquid superheat limit. Heating this additional volume decreases the efficiency of the device and significantly increases the level of residual heat present after firing. If this additional heat cannot be removed between successive firings of the nozzle, the ink in the nozzles will boil continuously, causing the nozzles to cease ejecting droplets in the intended manner.
The primary cooling mechanism of printheads on the market is currently thermal conduction, with existing printheads implementing a large heat sink to dissipate heat absorbed from the printhead chip. The ability of this heatsink to cool the liquid in the nozzles is limited by the thermal resistance between the nozzles and the heatsink and by the heat flux generated by the firing nozzles. As the extra energy required to heat the protective layers of a coated heater contributes to an increased heat flux, more severe constraints are imposed on the density of the nozzles on the printhead and the nozzle firing rate. This in turn has an impact on the print resolution, the printhead size, the print speed and the manufacturing costs.
The Applicant has developed a range of printheads without protective coatings added to the heaters to lower the energy needed to form a vapor bubble. These heaters form a thin surface oxide layer that has an oxygen diffusivity low enough to slow the rate of further oxidation to a level that gives the printhead an acceptable lifetime. However, the oxide layer does grow with time and in particular, with the number of drive pulses or actuations sent to the heater. This changes the heater resistance over the operation lifetime of the heater and therefore the drop ejection characteristics. It will be appreciated that this can be detrimental to print quality.